When do the mesoscales matter for trade cumulus clouds? A EUREC4A Perspective

Shallow cumulus clouds in trades present a unique challenge because they are both difficult to observe and describe. Recent studies have shown that mesoscale dynamics and synoptic scale cloud-controlling factors are important to describe them, but the interplay between these two scales is not well understood. Therefore, we attempt to disentangle their interplay to determine when mesoscale dynamics become important for the clouds. We have utilized data from the EUREC4A campaign aided with satellite and reanalysis data to extensively describe the cloudiness observed during the same period. 

We find a bimodal distribution of cloud top height from WALES LIDAR, with peaks at 1 km and 2 km for low and high clouds, respectively. A multiple linear regression analysis of six cloud-controlling factors (CCF) explains 80% of the variance in projected cloud cover. Further analysis reveals that the wind shear in the cloud layer is the leading CCF for low cloud fraction and mesoscale vertical motion is not a strong control for such low clouds not under high clouds. For higher clouds, mesoscale vertical motion at 1900m is the leading factor with a 0.68 correlation to high cloud cover. Furthermore, rain co-varies with high cloud cover substantially, on many days, thus we conclude that mesoscale dynamics are more important for high clouds. 

Alongside, ERA5 shows that variability in vertical motion is associated with high cloud variability only during the periods around rain. Thus, it becomes important to study high clouds in a cloud-circulation-rain framework. We propose a life cycle hypothesis to study various aspects of the coupling between mesoscale dynamics and trade wind clouds. We hypothesize that different stages of the life cycle of mesoscale cloud fraction of high clouds are related to growth by cloud-circulation coupling and decay by rain/cold pools driven circulations.